A note on the Gauge Symmetries of Unimodular Gravity

The symmetries of Unimodular Gravity are clarified somewhat.Comment: 4 pages, v2: acknowledgments correcte

Unimodular Gravity and General Relativity UV divergent contributions to the scattering of massive scalar particles

We work out the one-loop and order $\kappa^2 m_\phi^2$ UV divergent contributions, coming from Unimodular Gravity and General Relativity, to the S matrix element of the scattering process $\phi + \phi\rightarrow \phi + \phi$ in a $\lambda \phi^4$ theory with mass $m_\phi$. We show that both Unimodular Gravity and General Relativity give rise to the same UV divergent contributions in Dimension Regularization. This seems to be at odds with the known result that in a multiplicative MS dimensional regularization scheme the General Relativity corrections, in the de Donder gauge, to the beta function $\beta_{\lambda}$ of the $\lambda$ coupling do not vanish, whereas the Unimodular Gravity corrections, in a certain gauge, do vanish. Actually, we show that the UV divergent contributions to the 1PI Feynman diagrams which give rise to those non-vanishing corrections to $\beta_{\lambda}$ do not contribute to the UV divergent behaviour of the S matrix element of $\phi + \phi\rightarrow \phi + \phi$ and this shows that any physical consequence --such existence of asymptotic freedom due to gravitational interactions-- drawn from the value of $\beta_{\lambda}$ is not physically meaningful.Comment: 13 pages, 4 figure

Quantum Corrections to Unimodular Gravity

The problem of the cosmological constant appears in a new light in Unimodular Gravity. In particular, the zero momentum piece of the potential (that is, the constant piece independent of the matter fields) does not automatically produce a cosmological constant proportional to it. The aim of this paper is to give some details on a calculation showing that quantum corrections do not renormalize the classical value of this observable.Comment: 34 page

Anomaly freedom in Seiberg-Witten noncommutative gauge theories

We show that noncommutative gauge theories with arbitrary compact gauge group defined by means of the Seiberg-Witten map have the same one-loop anomalies as their commutative counterparts. This is done in two steps. By explicitly calculating the \epsilon^{\m_1\m_2\m_3\m_4} part of the renormalized effective action, we first find the would-be one-loop anomaly of the theory to all orders in the noncommutativity parameter \theta^{\m\n}. And secondly we isolate in the would-be anomaly radiative corrections which are not BRS trivial. This gives as the only true anomaly occurring in the theory the standard Bardeen anomaly of commutative spacetime, which is set to zero by the usual anomaly cancellation condition.Comment: LaTeX 2e, no macros, no figures, 32 A4 page

Noncommutative QCD, first-order-in-theta-deformed instantons and 't Hooft vertices

For commutative Euclidean time, we study the existence of field configurations that {\it a)} are formal power series expansions in h\theta^{\m\n}, {\it b)} go to ordinary (anti-)instantons as h\theta^{\m\n}\to 0, and {\it c)} render stationary the classical action of Euclidean noncommutative SU(3) Yang-Mills theory. We show that the noncommutative (anti-)self-duality equations have no solutions of this type at any order in h\theta^{\m\n}. However, we obtain all the deformations --called first-order-in-$\theta$-deformed instantons-- of the ordinary instanton that, at first order in h\theta^{\m\n}, satisfy the equations of motion of Euclidean noncommutative SU(3) Yang-Mills theory. We analyze the quantum effects that these field configurations give rise to in noncommutative SU(3) with one, two and three nearly massless flavours and compute the corresponding 't Hooft vertices, also, at first order in h\theta^{\m\n}. Other issues analyzed in this paper are the existence at higher orders in h\theta^{\m\n} of topologically nontrivial solutions of the type mentioned above and the classification of the classical vacua of noncommutative SU(N) Yang-Mills theory that are power series in h\theta^{\m\n}.Comment: Latex. Some macros. No figures. 42 pages. Typos correcte

First Light LBT AO Images of HR 8799 bcde at 1.65 and 3.3 Microns: New Discrepancies between Young Planets and Old Brown Dwarfs

As the only directly imaged multiple planet system, HR 8799 provides a unique opportunity to study the physical properties of several planets in parallel. In this paper, we image all four of the HR 8799 planets at H-band and 3.3 microns with the new LBT adaptive optics system, PISCES, and LBTI/LMIRCam. Our images offer an unprecedented view of the system, allowing us to obtain H and 3.3$ micron photometry of the innermost planet (for the first time) and put strong upper-limits on the presence of a hypothetical fifth companion. We find that all four planets are unexpectedly bright at 3.3 microns compared to the equilibrium chemistry models used for field brown dwarfs, which predict that planets should be faint at 3.3 microns due to CH4 opacity. We attempt to model the planets with thick-cloudy, non-equilibrium chemistry atmospheres, but find that removing CH4 to fit the 3.3 micron photometry increases the predicted L' (3.8 microns) flux enough that it is inconsistent with observations. In an effort to fit the SED of the HR 8799 planets, we construct mixtures of cloudy atmospheres, which are intended to represent planets covered by clouds of varying opacity. In this scenario, regions with low opacity look hot and bright, while regions with high opacity look faint, similar to the patchy cloud structures on Jupiter and L/T transition brown-dwarfs. Our mixed cloud models reproduce all of the available data, but self-consistent models are still necessary to demonstrate their viability.Comment: Accepted to Ap